管道时陷阱是否按预期工作？

时间：2014-03-06 10:26:49

标签： bash pipe subshell bash-trap

以下是问题演示的最小代码： http://pastebin.com/5TXDpSh5 

#!/bin/bash
set -e
set -o pipefail

function echoTraps() {
    echo "= on start:"
    trap -p
    trap -- 'echo func-EXIT' EXIT
    echo "= after set new:"
    trap -p
    # we can ensure after script done - file '/tmp/tmp.txt' was not created
    trap -- 'echo SIG 1>/tmp/tmp.txt' SIGPIPE SIGHUP SIGINT SIGQUIT SIGTERM
}

trap -- 'echo main-EXIT1' EXIT

echo "===== subshell trap"
( echoTraps; )

echo "===== pipe trap"
echoTraps | cat

echo "===== done everything"

输出

===== subshell trap
= on start:
= after set new:
trap -- 'echo func-EXIT' EXIT
func-EXIT
===== pipe trap
= on start:
= after set new:
trap -- 'echo func-EXIT' EXIT
===== done everything
main-EXIT1

预期产出

===== subshell trap
= on start:
= after set new:
trap -- 'echo func-EXIT' EXIT
func-EXIT
===== pipe trap
= on start:
= after set new:
trap -- 'echo func-EXIT' EXIT
func-EXIT                 <---- here is the expected difference
===== done everything
main-EXIT1

注意：我测试了OSX 10.9.2 bash（3.2.51） - 其他版本的bash在实际预期输出之间有相同的差异，并描述了bellow

2 个答案:

答案 0 :(得分：1)

了解这种行为是否是预期的唯一方法是询问Chet Ramey（GNU bash维护者）。请将您的报告发送至bug-bash@gnu.org

您可以看到当前行为似乎是正确的，因为它明确处理了子shell案例：http://git.savannah.gnu.org/cgit/bash.git/tree/execute_cmd.c#n621 

/* We want to run the exit trap for forced {} subshells, and we
   want to note this before execute_in_subshell modifies the
   COMMAND struct.  Need to keep in mind that execute_in_subshell
   runs the exit trap for () subshells itself. */
/* This handles { command; } & */
s = user_subshell == 0 && command->type == cm_group && pipe_in == NO_PIPE && pipe_out == NO_PIPE && asynchronous;
/* run exit trap for : | { ...; } and { ...; } | : */
/* run exit trap for : | ( ...; ) and ( ...; ) | : */
s += user_subshell == 0 && command->type == cm_group && (pipe_in != NO_PIPE || pipe_out != NO_PIPE) && asynchronous == 0;

last_command_exit_value = execute_in_subshell (command, asynchronous, pipe_in, pipe_out, fds_to_close);
if (s)
    subshell_exit (last_command_exit_value);
else
    sh_exit (last_command_exit_value);

如您所见，显式子shell情况作为特殊情况处理（命令分组也是如此）。由于多个错误报告，Adrian发现，这种行为在历史上已经发生了变化。

这是此特定功能的更改列表（触发子shell上的EXIT陷阱）：

提交：http://git.savannah.gnu.org/cgit/bash.git/commit/?id=a37d979e7b706ce9babf1306c6b370c327038eb9 

+execute_cmd.c
+ - execute_command_internal: make sure to run the EXIT trap for group
+   commands anywhere in pipelines, not just at the end.  From a point
+   raised by Andreas Schwab <schwab@linux-m68k.org>

报告：https://lists.gnu.org/archive/html/bug-bash/2013-04/msg00126.html（Re：在等待期间未触发管道子shell中的陷阱EXIT）

提交：http://git.savannah.gnu.org/cgit/bash.git/commit/?id=1a81420a36fafc5217e770e042fd39a1353a41f9 

+execute_cmd.c
+ - execute_command_internal: make sure any subshell forked to run a
+   group command or user subshell at the end of a pipeline runs any
+   EXIT trap it sets.  Fixes debian bash bug 698411
+   http://bugs.debian.org/cgi-big/bugreport.cgi?bug=698411

报告：https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=698411（退出陷阱和管道和子shell）

提交：http://git.savannah.gnu.org/cgit/bash.git/commit/?id=fd58d46e0d058aa983eea532bfd7d4c597adef54 

+execute_cmd.c
+ - execute_command_internal: make sure to call subshell_exit for
+   {} group commands executed asynchronously (&).  Part of fix for
+   EXIT trap bug reported by Maarten Billemont <lhunath@lyndir.com>

报告：http://lists.gnu.org/archive/html/bug-bash/2012-07/msg00084.html（交互式shell中的EXIT陷阱）

最近还有一个关于EXIT陷阱的bug报告没有在某些预期的上下文中执行：http://lists.gnu.org/archive/html/bug-bash/2016-11/msg00054.html

答案 1 :(得分：0)

以下是一些适合您娱乐的测试案例：

$ cat traps.sh
#!/bin/bash

echoTraps() {
        echo "entering echoTraps()"

        printf "  traps: %s\n" "$(trap -p)"

        echo "  setting trap"
        trap -- 'echo "func-exit()"' EXIT

        printf "  traps: %s\n" "$(trap -p)"

        echo "exiting echoTraps()"
}

trap -- 'echo "main-exit()"' EXIT

echo "===== calling '( echoTraps; )'"
( echoTraps; )
echo

echo "===== calling 'echoTraps | cat'"
echoTraps | cat
echo

echo "===== calling '( echoTraps; ) | cat'"
( echoTraps; ) | cat
echo

echo "===== calling '{ echoTraps; } | cat'"
{ echoTraps; } | cat
echo

的bash-4.2.25（1）

$ ./traps.sh
===== calling '( echoTraps; )'
entering echoTraps()
  traps:
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()
func-exit()

===== calling 'echoTraps | cat'
entering echoTraps()
  traps: trap -- 'echo "main-exit()"' EXIT
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()

===== calling '( echoTraps; ) | cat'
entering echoTraps()
  traps:
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()
func-exit()

===== calling '{ echoTraps; } | cat'
entering echoTraps()
  traps: trap -- 'echo "main-exit()"' EXIT
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()

main-exit()

的bash-4.3.0（1）

$ bash-static-4.3.2/bin/bash-static traps.sh
===== calling '( echoTraps; )'
entering echoTraps()
  traps:
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()
func-exit()

===== calling 'echoTraps | cat'
entering echoTraps()
  traps: trap -- 'echo "main-exit()"' EXIT
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()

===== calling '( echoTraps; ) | cat'
entering echoTraps()
  traps:
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()
func-exit()

===== calling '{ echoTraps; } | cat'
entering echoTraps()
  traps: trap -- 'echo "main-exit()"' EXIT
  setting trap
  traps: trap -- 'echo "func-exit()"' EXIT
exiting echoTraps()
func-exit()

main-exit()

底线：不要依赖这样的边缘情况。我记得调查了关于子壳和管道的其他不一致（不是关于陷阱），并试图绕着bash源代码包围你并且你不想沿着这条路走下去并试图理解为什么它表现得像它确实在某些情况下（代码非常可怕，顺便说一下）。正如你所看到的，有些东西似乎已经“修复”了/和/但我的两个例子都表现得与你的不同。

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